SN1 mechanism

The second major type of nucleophilic substitution mechanism is the SN1 mechanism. This mechanism proceeds via two steps. The first step (the slow step) involves the breakdown of the alkyl halide into an alkyl carbocation and a leaving group anion. The second step (the fast step) involves the formation of a bond between the nucleophile and the alkyl carbocation

Because the activated complex contains only one species—the alkyl carbocation—the substitution is considered unimolecular.
Carbocations contain sp2 hybridized orbitals and thus have planar structures. SN1 mechanisms proceed via a carbocation intermediate, so a nucleophile attack is equally possible from either side of the plane. Therefore, a pure, optically active alkyl halide undergoing an SN1 substitution reaction will generate a racemic mixture as a product, as shown in Figure .